A prior art disk drive 10, see FIG. 1, typically includes a main integrated circuit, which is typically called a system on a chip (SOC) 11 that contains many of the electronics and firmware for the drive. Each disk (not shown) can have thin film magnetic material on each of the planar surfaces. Each recording surface normally has a dedicated pair of read and write heads packaged in a slider 13 that is mechanically positioned over the rotating disk by an actuator (not shown). The actuators also provide the electrical connections to the slider components. The actuators also contain the arm electronics (AE) chip 12 which typically include preamps for the read heads, write drivers and fly-height controls. A flex cable connects the SOC 11 to the AE 12. The AE typically include digital and analog circuitry that control the signals sent to components in the slider and process the signals received from the slider components. The AE can include registers that are set using serial data from the SOC to provide parameters for the AE functions. The write driver generates an analog signal that is applied to the inductive coil in the write head to write data by selectively magnetizing portions of the magnetic material on the surface of the rotating disk.
As the disk rotates under the slider, the slider is said to “fly” above the disk although the spacing is quite small and contact can occur. Controlling the fly-height is an important part of the design. Fly-height control circuitry 14 interfaces with fly-height components 15 in the slider. Thermal fly-height control (TFC) is one prior art control technique that uses a heater element (not shown) disposed in the slider. The fly-height can be adjusted by heating the slider with the heater. Electrical current supplied to the heater by fly-height control circuitry 14 generates heat to thermally expand the slider and modulate the fly-height. The fly-height components 15 can also include other elements in addition to the heater.
Published U.S. patent application 20120099218 by Kurita, et al. (Apr. 26, 2012) describes a magnetic-recording head used in a hard disk drive with a thermal fly-height control (TFC) element and an embedded contact sensor element configurable as a second TFC element. The heater element is configured as a TFC element to coarsely adjust the fly-height. The embedded contact sensor is configured to detect contact with the magnetic-recording disk, and to function as a second heater element that provides fine adjustment of the fly-height.
A fly-height control system can also include embedded contact sensors (ECS) 17 in the slider along the associated ECS Control circuitry 16 in the AE. Resistor temperature detectors (RTD) in the sliders have been used in the prior art to determine when the read/write head makes physical contact with the magnetic-recording disk based upon changes in the temperature of the slider when contact occurs. RTD architectures can use a single temperature sensor that measures temperature based on the amount of voltage across a single temperature sensor. Published U.S. patent application 20130176643 by Contreras, et al. Jul. 11, 2013, which is commonly assigned with the present application, describes a distributed temperature detector architecture in a head disk interface system of a hard-disk drive (HDD). The slider can include a first temperature sensor that is located relatively near an air bearing surface (ABS) and a second temperature sensor that is offset from the ABS. The read/write IC is configured to detect when the slider makes physical contact with a disk based on a difference in temperature measured by the first and second temperature sensor. The first and second temperature sensors form a bridge circuit, such as a Wheatstone bridge, with a first IC resistor and a second IC resistor that both reside in the read/write IC, allowing the temperature of the read/write head to be accurately measured.
A balanced embedded contact sensor with low noise architecture is described in published U.S. patent application 20130163110 which is commonly assigned with the present application and listed as a related application above. In one embodiment a slider having a resistive temperature detector (RTD) is used with AE circuitry that includes a balance resistor having the same resistance as the RTD when the slider is not in physical contact with the disk. Equal current flows through the RTD and the balance resistor to allow the voltage change across the RTD to be used as a measure of physical contact between the head slider. In an alternate embodiment, the slider includes two RTDs connected in series, and the balance resistor may possess the same resistance as the two series RTDs. The two RTDs may be designed to vary inversely with environmental changes to avoid the need to recalibrate the balance resistor after any environmental change.
A balanced amplifier with DC capability and low noise is needed for a balanced Embedded Contact Sensor (bECS) system. However, temperature changes in ECS in the slider due to write currents, TFC operation, and ambient temperature fluctuations, can add an undesired offset to the ECS tracking signal which is not properly balanced in and which limits the gain of the bECS amplifier.